Here's the microbe that's wreaking havoc on Atlantic oysters

Lovers of raw oysters used to wait past August for the cooler "R" months. Now, summertime sales of oysters are reaching new highs. This year, so are the food-borne illnesses they carry.

The two things aren't necessarily connected. Rather, it's a toxic mixture of warm temperatures, heavy rain and – perhaps – inadequate regulation that has precipitated shellfishery shutdowns in six states, four of them in the Northeast. In every state, Vibrio parahaemolyticus is to blame.

Vibrio parahaemolyticus (pronounce it para-hemo-litikus) is a bacterium commonly found in brackish waters where oysters and clams also thrive. It's a cousin of cholera, but most vibrios are harmless. V. parahaemolyticus causes gastroenteritis, but it's rarely dangerous. It's not frequently treated. It's not often even reported.

That's changing, which may contribute to a higher number of vibrio outbreaks. Their severity and span in 2013 is "historically new," said Vaughn Cooper, an associate professor at University of New Hampshire who studies the bacteria. Better detection is a factor, but so is climate change, he said: Warmer waters and heavier rains lead to higher levels of vibrio bacteria.

"One overall question that we've been interested in is whether climate change is just a rising tide lifting all (bacterial) boats or is it just particularly increasing the frequency of these pathogenic strains," Cooper said.

The latest vibrio shutdown and recall of oysters hit Martha's Vineyard two days ago. Oyster operations in the South Shore towns of Duxbury, Marshfield and Plymouth, Mass., have been shut down since Aug. 30. Vibrio has also hit oyster beds in Connecticut, New Jersey, New York, Virginia and Washington state.

"It's been a very disappointing summer," said Lori Howell, co-owner of Spinney Creek Shellfish, a distributor in Eliot, Me. "In 30 years we have never done a recall, and it's terrible to do a recall."

Duxbury-based Island Creek Oysters was among 40 Plymouth-area shellfish farms that issued a recall in August. "In the coming weeks, Island Creek Oysters will work with state health officials to ensure that normal water temperatures have completely eliminated the risk of foodborne illness from these areas," the company said in a statement at the time.

In April, Massachusetts fisheries officials promulgated new permit conditions and a "vibrio control plan." Oyster farmers are now required to keep the shellfish shaded from sun immediately after harvest, and to put them on ice once they reach land. The shellfish must be cooled to an internal temperature of 50 degrees Farenheit or less, within 10 hours.

A spokeswoman for the Massachusetts Division of Marine Fisheries said Bay State shellfish operations are complying with the new regulations. Nonetheless, outbreaks are higher than years past.

In fact, Massachusetts may be behind the curve when it comes to regulating how quickly oysters are cooled. At the biennial meeting of the Interstate Shellfish Sanitation Conference in Texas next month, that's likely to be a topic of heated discussion.

When an oyster comes out of the water, it closes up. Kept warm, it's like an incubator for whatever is living inside, said Ken Moore, executive director of the ISSC. "Most states are operating you have to get to 50 (degrees) within five hours," he said. "Now it might be one hour."

Further out, researchers would like to be able to predict what conditions bring elevated risk of vibrio contamination. Algal blooms, like the "red tide" that sometimes closes Massachusetts beaches to swimming, may harbor vibrios, said UNH's Cooper. Heavy rainfall may also be linked to elevated risk, he said – but at this point it's hard to predict when a vibrio outbreak will occur, especially in temperate northern waters such as these.

"I'll be honest, I still eat them," Cooper said of raw oysters. "At this point it's the spoils of the research."